1. Field of the Invention
The present invention relates to a process for preparing esters and carboxylic acids from C.sub.2 or higher olefins, preferably propylene, utilizing relatively mild reaction conditions and facile recovery procedures, and, in a further aspect, to a continuous process for such preparation.
2. Description of Related Patents
The conversion of propylene to its corresponding carboxylic acid or esters is an important process for the chemical manufacturing industry. For example, it has long been desired to find a convenient method for preparing high octane motor fuels and we have discovered that esters such as methylisobutyrate, which may be prepared from propylene, can be employed as a minor or major component of such motor fuels. In addition, methylisobutyrate may be employed as a solvent or as an intermediate in the synthesis of methylmethacrylate.
The Koch reaction, normally used to prepare carboxylic acids, provides a known path for the manufacture of esters or carboxylic acids when olefins are reacted with carbon monoxide and an alcohol or water in the presence of an acid such as sulfuric or phosphoric acid, hydrogen fluoride or boron trifluoride:water. The reaction sequence can be represented in general as follows: ##STR1##
Generally, the Koch reaction is promoted when the acid is used in considerable excess over the amount of olefin or alcohol (or water) and the temperature is kept relatively low. A major disadvantage of this preparative route, however, is that the conversion of ethylene and propylene, which are less reactive olefins than branched olefins, to their respective ester or acid forms requires temperatures of 100.degree. to 150.degree. C. and pressures of 200-1000 atm, as disclosed in U.S. Pat. No. 2,378,009 and in J. R. Roland, et al., J.A.C.S., 72, 2122 (1950).
It is known from S. Pawlenko, Chem. Ing.-Techn., 40, 52 (1968) that oxonium tetrafluoroborates of the type (ROH.sub.2).sup.+ (BF.sub.4).sup.- are catalysts for the batch esterification of olefins such as ethylene and propylene, which are ordinarily difficult to carboxylate, at 40.degree.-50.degree. C. and 100-150 atm total pressure.
U.S. Pat. No. 4,262,138 discloses the production of methylisobutyrate from propylene using an equimolar mixture of BF.sub.3 to methanol generally under a pressure of 10-300 atm. Because the product is not easily recoverable (the BF.sub.3 complexes therewith), the patent describes an elaborate distillation procedure for separating the acid from the ester.
It was discovered by Y. Matsushima, et al., Chem. Letters, 433 (1973) that carbon monoxide is absorbed by cuprous ion in BF.sub.3 --water systems under atmospheric pressure. Based on this discovery it was reported by Y. Souma, Osaka Kogyo Gijutsu Shikensho Hokoku, 79, 33b, 1-76 (1977) that in the presence of copper I or silver I salt the Koch reaction may be conducted at atmospheric pressures to convert linear olefins higher than propylene or branched olefins such as isobutylene to their respective carboxylic acids and esters. The copper I or silver I salt effectively increases the carbon monoxide concentration in the acid solution and leads to facile trapping of cationic intermediates involved in the reaction.